The invention relates to a headlight or a light for a motor vehicle, with a diffusion lens, a casing, light sources arranged in the casing, reflectors allocated to said light sources and a ventilation system, which comprises at least one air intake opening, an air outlet opening and a blower.
An embodiment of this kind is disclosed by FR 2 701 756 A1. The blower built into the headlight casing sucks fresh air in via an air intake opening provided in the rear wall of the casing and forces this air along the lower wall of the casingxe2x80x94guided by air guiding devicesxe2x80x94against the diffusion lens, along which the air flows in order to flow away to the outsidexe2x80x94again guided by air guiding devicesxe2x80x94along the upper wall of the casing and through an upper air outlet opening, provided in the region of the rear wall of the casing. The air exchange brought about in this way is intended to remove moist air from the headlight, in order to prevent condensing of the moisture contained in the air on the diffusion lens when the latter or the entire headlight cools down.
In the case of headlights or lights of the new generation, a clear lens is used. The prescribed light distribution is then achieved by deliberate geometrical design of the reflectors. In addition to their actual function, that is of reflecting the light in the direction of light emission, the reflectors can also be used as carriers for light emitting optics for additional lights, such as a flashing turn-signal or fog lights for example. On the engine compartment side, the headlight or light is then enclosed by a casing. By contrast with conventional headlights or lights, here there are, for example, three or even four light sources arranged in one headlight. This simplifies assembly, since, by fitting the headlight, all the lights are already attached to the vehicle. Furthermore, this gives the headlight a technically very interesting impression for a viewer from the outside. By accommodating so many light sources on a relatively small surface area, however, new problems arise with regard to the development of heat. For instance, temperatures of up to 230 degrees are not exceptional in such a headlight. This is of great significance in particular when the vehicle is at a standstill, that is to say there is no longer any cooling by the relative wind. The development of heat may cause, for example, deformations of the diffusion lens or other plastic parts.
In the case of headlights or lights of the new generation, plastics, in particular PCxe2x80x94polycarbonate, UPxe2x80x94unsaturated polyester, PP-GFxe2x80x94glass fiber reinforced polypropylene (ester) and PVCxe2x80x94polyvinyl chloride, are customarily used for the casing or the reflector, or for sealing means. These are fogging plastics, i.e. the plastics contain gassing-out volatile constituents. Constituents which fog out from such plastics are, for example, plasticizers, amines (PU foam catalysts), lubricants, stabilizers, flame retardants or solvents. An exact listing of the fogging constituents is listed in xe2x80x9cDas Foggingproblem: Messmethoden, Wege und Erfolgexe2x80x9d [the fogging problem: measuring methods, approaches and successes], ATZ Automobiltechnische Zeitschrift 96 (94) pages 238-246 Table 4. The fogging leads to the diffusion lens of the reflectors misting over or turning gray or else to decomposing of the plastic. The fogging increases exponentially with temperature; this is described for example in xe2x80x9cTemperaturabhangigkeit des Fogging-Phanomensxe2x80x9d [temperature dependence of the fogging phenomenon] Kunststoffe 83 (1993), by F. Loock, Th. Lampe, A. M. Bahadir.
The invention is based on the object of preventing the risk of the lens or the reflectors in a headlight or a light of the new generation from turning gray or misting over due to fogging.
On the basis of the headlight or light described at the beginning, this object is achieved according to the invention by at least one of the headlight parts consisting at least partially of a fogging plastic, by the blower being a suction blower sucking air away out of the casing, and by air guiding devices being arranged within the casing in such a way that the blower guides the air away from the lens and deliberately past heat pockets to the rear side of the casing, facing away from the lens.
The sucking away of the air has the effect that the components around which the flow passes, that is in particular the reflectors and the casing, are cooled, so that the constituents fogging out from these parts are reduced. The air guidance provided according to the invention also has an effect on the amount of fogging condensate in the headlight. Guiding the air flow past the reflector is particularly important, since the latter requires particular protection against depositing of condensates due to its complicated geometrical design, in order to ensure or maintain unchanged the prescribed light distribution. It is important, furthermore, that the blower guides the air away from the lens, since, if blown air were directed onto the windshield, volatile constituents would be blown onto the lens.
The air flow guided in a way according to the invention causes the fogging plastic parts to be cooled, so that a smaller amount of volatile constituents is released from the plastic parts. The remaining, still volatile constituents are carried away out of the headlight by the increased air exchange. It is important for this purpose that the air flow flowing from the air intake opening to the air outlet opening is increased adequately.
It is expedient if the blower is arranged on or in a cover which releasably closes an opening in the rear side of the casing. In headlights of the generic type, a cover of this kind is generally provided in order to be able to exchange the light sources easily, without having to remove the entire headlight for this purpose. The cover further serves as a fastening attachment for a headlight-range adjusting device or other electrical add-on parts. The arrangement of the blower on or in this cover allows headlights of the new generation to be retrofitted quickly and easily by exchanging the cover for a cover fitted with a blower.
To protect the reflector, it is expedient if the air guiding devices comprise through-flow openings in the reflector, these through-flow openings preferably being arranged close to the light source. In addition, it is advantageous if the air guiding devices comprise gaps between the outer rim of the reflector and the wall of the casing.
The blower is expediently electrically coupled in its function to the parking lights. As a result, when the parking lights are switched on, the blower is activated at the same time, and consequently switched on even when the vehicle is stationary, to be able to compensate for the lack of headlight cooling by the relative wind now absent.
It is further proposed that it should be possible. for the blower to be switched off after a prescribed further running time after the switching off of the headlight or light. As a result, the fully heated-up headlight is still cooled for a time even when the vehicle is stationary, and fogging is prevented.
If the headlight has a plurality of light sources of different light intensity, it is expedient to allocate the blower to the light source of the strongest light intensity, since this emits most heat and consequently represents the most critical point in the headlight. This blower arrangement achieves particularly effective heat dissipation.
The invention is explained in more detail below on the basis of an exemplary embodiment.